1. Field of the Invention
The present invention generally relates to a fixing device, an image forming apparatus, and a fixing method. More particularly, the present invention relates to a fixing device using a fixing liquid for dissolving or swelling toner, an image forming apparatus including the fixing device, and a fixing method using the fixing liquid for dissolving or swelling toner.
2. Description of the Related Art
An image forming apparatus such as a printer, a facsimile machine, a copier and the like forms images including characters and symbols on a recording medium such as paper, fiber, and a plastic sheet based on image information. Especially, an image forming apparatus using the electrophotographic method has been widely used because high-resolution images can be rapidly formed. In the image forming apparatus using the electrophotographic method, a fixing device using a heat fixing method is widely used where toner on the recording medium is heated to be melted, so that the melted toner is pressed to be fixed onto the recording medium. In this case, more than half of the power consumption is used for heating the toner.
On the other hand, recently, in the viewpoint of the countermeasures to the environmental problems, there has been a strong demand for reducing the power consumption (energy saving) of the image forming apparatuses. To that end, a fixing method used in the fixing device is provided for drastically reducing the temperature for heating toner or another fixing method used in the fixing device is provided where it is not necessary to heat toner (unheated fixing method). Especially, the unheated fixing method where toner is fixed onto the recording medium without heating any toner is more advantageous from the viewpoint of low power consumption.
For example, Japanese Patent No. 3290513 (Patent Document 1) discloses a wet-type toner fixing method where an oil droplet-in-water type fixing agent, in which an organic compound being capable of dissolving or swelling toner and being insoluble or difficult to be dissolved in water is dispersed and mixed in water, is sprayed or dropped onto a surface of a substrate to be subjected to fixing on which unfixed toner is arranged at a predetermined position so that the toner is dissolved or swelled, and subsequently the substrate to be subjected to fixing is dried out.
Further, Japanese Patent No. 4302700 (Patent Document 2) discloses a fixing device that applies a fixing agent onto resin particles formed on a recording medium based on the image information to be imaged, the fixing agent including softener softening resin particles including resin by dissolving or swelling at least a part of the resin. The fixing device includes a foam-like solution generating unit forming the foam-like fixing agent, a film thickness control unit controlling the film thickness of the foam-like fixing agent generated by the foam-like solution generating unit, and an application unit applying the foam-like fixing agent having the controlled thickness to the resin particles on the recording medium.
Japanese Patent Application Publication No. 2009-69256 (Patent Document 3) discloses a temperature holding device and an image forming apparatus including the temperature holding device, the temperature holding device including a container entirely or partially made of a thermal storage member for containing a fixing liquid dissolving or swelling toner, a heating unit disposed in a main body of the image forming apparatus and heating the thermal storage member of the temperature holding device, a liquid temperature detection unit detecting the temperature of the fixing liquid in the container, a control unit controlling the heat quantity transferred from the heating unit to the thermal storage member based on the detection result by the liquid temperature detection unit, and a fixing liquid fixing unit applying the fixing liquid stored in the container so that the toner on the recording medium is fixed onto the recording medium.
However, when the wet-type toner fixing method disclosed in Patent Document 1 is used, the oil droplet-in-water type fixing agent in which an organic compound being capable of dissolving or swelling toner and being insoluble or difficult to be dissolved in water is dispersed and mixed in water is used. Because of this feature, when a large amount of fixing agent is applied to unfixed toner, a recording medium (an object on which the fixing agent is to be fixed) such as a transfer sheet may absorb water included in the fixing agent so as to crinkle or curl. As a result, a stable and quick feeding recording medium which is necessary in the image forming apparatus may be degraded.
To resolve the problem, a drying device may be provided so as to evaporate the large amount of water included in the fixing agent and having been applied to the recording medium. In this method, however, power comparable to the power consumption of an image forming apparatus using the heat fixing method may be necessary.
Further, in a case where a fixing liquid is applied to a unfixed toner layer on the recording medium by using an application roller, when the fixing liquid layer on the application roller is thinner than the unfixed toner layer, at the position where the application roller is separated from the recording medium, unfixed toner particles are pulled by the surface tension generated by the liquid film of the fixing liquid on the surface of the application roller. As a result, a so-called toner offset where toner particles are adhered to the surface of the application roller may be generated, thereby remarkably deteriorating the image on the recording medium.
On the other hand, when the fixing liquid layer on the application roller is sufficiently thicker than the unfixed toner layer, at the position where the application roller is separated from the recording medium, the surface tension generated by the liquid film of the fixing liquid on the surface of the application roller may make it hard for the fixing liquid to be directly applied to the toner particles due to there being too much liquid. As a result, it may be difficult for the toner to be transferred to the roller side but a large amount of fixing liquid is applied to the sheet surface. Because of the features, the toner particles may be swept onto the recording medium due to excessive fixing liquid, or drying time is increased, so that fixing reliability may be degraded. Further, in this case, the recording medium may remain in a wet state (i.e., when the recording medium is touched, the wet feeling is recognized). Further, in a case where the fixing liquid includes water, when a large amount is applied to the recording medium including cellulose such as paper, the recording medium may remarkably curl, thereby causing a paper jam when the recording medium is fed in an apparatus such as an image forming apparatus.
On the other hand, in order to improve the fixing reliability, reduce the wet feeling, and prevent the curling, it may be necessary that only a slight amount of fixing liquid is applied to the toner layer on the recording medium. However, when the wet-type toner fixing method disclosed in Patent Document 1 is used, it may be very difficult to achieve a good balance between the slight amount of application of the fixing liquid to the toner layer on the recording medium and the prevention of the toner offset to the application roller. Further, even in a case where a daicoat unit, a blade application unit, or a wire-bar application unit is used as a contact application unit, when the amount of fixing liquid is small, toner may be adhered to the contact application unit due to the surface tension, which may degrade the image quality.
On the other hand, in the viewpoints of eliminating the printing failures including the toner offset and reducing the consumption amount of the fixing liquid, it may be necessary to stably supply the fixing liquid to the recording medium regardless of the usage environment.
However, when the fixing device disclosed in Patent Document 2 or the image forming apparatus disclosed in Patent Document 3 is used, it may be possible to achieve a good balance between the slight amount of application of the fixing liquid to the toner layer on the recording medium and the prevention of the toner offset to the application roller. However, in those inventions, no environmental fluctuation is considered. Therefore, for example, when moisture (moisture content, water amount) in the recording medium changes due to atmospheric humidity change, the penetration rate of the fixing liquid into the recording medium changes accordingly. As a result, due to the change of the penetration rate of the fixing liquid, the fixing characteristic may vary.
Namely, when the atmospheric humidity is low, a larger amount of fixing liquid is absorbed into the recording medium even when the same amount of the fixing liquid is supplied as the amount of the fixing liquid when the atmospheric humidity is high. As a result, increased moisture (increased moisture content) in the recording medium may exceed the desired amount. On the other hand, when the atmospheric humidity is high, the increased moisture in the recording medium may be less than the desired amount.
Further, when the increased moisture in the recording medium is large, a curl or crinkle may be generated. Further, when the increased moisture in the recording medium fluctuates, printing failure may occur depending on the fluctuation amount. Further, in continuous printing, if no control is performed on the increased moisture in the recording medium to be maintained constant, the fixing liquid more than necessary may be consumed, thereby increasing the frequency of replacing the consumable supply of the fixing liquid.